1. Field of the Invention
The invention generally relates to analytical methods and means; specifically, it relates to a method of monitoring the concentration of a gas in a fluid medium; further, the invention relates to an apparatus for carrying out the method combined or not with controlling the concentration of a gas in a fluid medium.
2. Prior Art
Numerous methods for monitoring gas concentrations are known and are frequently classified according to the measuring principles involved and/or the characteristic means used for separating the substance of interest from a mixture.
Accordingly, when using a membrane that is permeable for the substance of interest but impermeable to other components of the mixture, the term "membrane-based" analysis is appropriate. When membranes are used mainly because of their separating effect, e.g. between a gas, such as carbon dioxide, and a liquid, such as a carbonated beverage, maximum permeability of the membrane for the gas of interest is generally assumed to be desirable.
For example, a prior art membrane-based method of determining the concentration of carbon dioxide in a beverage, such as beer, as disclosed in EP 0 043 229 and EP 0 103 988, uses membranes that exhibit a maximum permeability for the gas of interest because the actual measuring parameter in these methods is the so-called equilibrium pressure that will build up within a membrane-enclosed chamber in response to the "concentration" (synonymous herein with "partial pressure") of the gas in a liquid that is "external" to the membrane-enclosed space. However, even with membranes made of silicone rubber which are notorious for their high permeability to gases, such as carbon dioxide, relatively long periods of time of typically in the order of 20 minutes may be required until equilibrium of gas pressures external and internal of the membrane-enclosed space is achieved. This method has been proposed for use in "controlling" the concentration of a gas, such as carbon dioxide, in a liquid, such as a beverage, i.e. maintaining a desired minimum concentration of the relatively costly carbon dioxide in a carbonated beverage by adding or "injecting" just those amounts of gas as are needed to maintain the required minimum concentration. Obviously, the results of such control will be determined to a large extent by the response and reliability of the measuring method used. And, while so-called "dynamic" pressure measuring methods are known, i.e. measuring a time-dependent pressure change and extrapolating the measured value or values towards infinity of time (and, hence, "perfect" equilibrium), such methods, though recommended for use in these prior art methods, tend to decrease reliability and reproducibility of the results for various reasons, e.g. because of mechanical deformations of the membrane under the impact of pressure and concurrent changes of chamber volume, membrane dimensions, permeability and other parameters that normally are expected to remain constant.